This invention relates to a process and solution for etching copper and copper oxides, and, more particularly, for selectively etching copper and copper oxides during the preparation of high density, multilayer interconnects.
The fabrication of a high density, multilayer interconnect often requires three components. They are the substrate materials, the interlayer dielectric, and the electrical conductor. To ensure system integrity, these materials must be compatible with each other as well as with VLSI devices. Copper and polyimide are often selected as the preferred conductor and interlayer dielectric, respectively. Copper is selected due to its low electrical resistance, high thermal conductivity, availability, and low cost. Polyimide is the selected interlayer dielectric due to its low dielectric constant, high thermal and chemical stability, good planarization characteristics, and ease of processing. However, poor macroscopic adhesion at the copper-polyimide interface is generally reported and attributed to the weak interface formation and islanding of copper on polyimide. In addition, the migration of copper-rich precipitates into polyimide can potentially change the dielectric properties of the polyimide. Consequently, an adhesion/diffusion barrier layer is usually placed between the copper and polyimide for long term reliability purposes. A variety of metal-polyimide systems have been investigated, with particular focus on chromium, titanium, nickel, and aluminum. To prevent delamination between the copper and polyimide on the sidewalls of the conductor features, a protective nickel overcoat can be used to form a barrier layer. Nickel may be selected due to its excellent corrosion resistance, and ease of low cost electrolytic plating.
Such metallized electrical interconnect substrates are typically prepared by sputtering an adhesion layer and then plating the interconnect on a polyimide surface. The metallization can include, for instance, a layer of chromium adjacent the polyimide, a layer of copper as the electrolytic plating interconnect, and a layer of titanium over the copper as a protective film. Photoresist is then spin coated and exposed to define the pattern for conductor and pillar plating. After electrolytic plating and stripping the photoresist, a thin layer of nickel overcoat is applied over the copper features to prevent corrosion and delamination problems. The substrate is then brought in contact with titanium, copper, and chromium etching solutions separately to remove those portions of the sputtered interconnect layers lying beneath the unexposed photoresist. The remaining unetched metallization will then form the desired electrical conductive network. As an example of such a process, reference is made to assignee's U.S. Pat. No. 4,810,332. As indicated therein, etching is a preferred subtractive process for copper removal.
Because, however, the protective nickel overcoat has a thickness in the range of a few microns, problems tend to arise when different etchants are in contact with nickel during the stripping process. Metal etchants with low selectivity may potentially attack the thin nickel overcoat thereby leaving portions of the underlying copper conductor unprotected. Such uncontrollability of the etching process is obviously undesirable since it can jeopardize fabrication yields as well as degrade the performance of the interconnects. Therefore, there is a need for an etching process which can selectively etch metals such as titanium, copper, and chromium without cross-attacking disimilar metals.
Copper etching is a well-known process in the printed circuit/electrical interconnect industry. The early etchants were often acid-based. For example, ferric chloride, chrome/sulfuric acid, hydrogen peroxide/sulfuric acid, and ammonium persulfate were predominant electronic grade etchants. A variety of these etching solutions are described in U.S. Pat. Nos. 2,982,625; 2,978,301; 4,401,509; 4,419,183; 4,437,931; 4,459,216; 4,462,861; 4,510,018; and 4,636,282.
Because of waste disposal and other problems with the acid-based etchants, alkaline-based solutions became the etchants of choice thereafter for many applications. These etchants most often were aqueous ammoniacal solutions containing carbonate ions and an oxidizing agent, such as sodium chlorite. Etchants of this type are described in U.S. Pat. Nos. 3,231,503 and 3,466,208. Both patents disclose etching solutions with comprise sodium chlorite, ammonium hydroxide and an ammonium salt, such as ammonium bicarbonate.
Other examples can be found of acid-based and alkaline-based copper etchants incorporating a variety of modifiers which achieve desirous advantageous properties. For instance, U.S. Pat. No. 3,514,408 describes an etchant which includes a film modifier, such as phthalic anhydride or phthalimide. U.S. Pat. No. 4,311,551 describes an etchant which includes an etch accelerating additive, such as cyanamide. Finally, U.S. Pat. No. 4,319,955 describes the effects of 5-nitro 1H indazole or pyrazole in combination with cupric ions, ammonium salt, ammonium hydroxide, and water.
More recently, certain new etching chemistries which are based on nitric solutions have been developed by Psi Star. U.S. Pat. Nos. 4,497,687; 4,545,850; and 4,632,727 describe such solutions which can improve the anisotropicity of copper etching with a specific crystal structure.
Thus, the forementioned references reflect the numerous copper etchant solutions to date. However, in the fabrication of high density interconnects, the existing etching techniques suffer several drawbacks: they are not sufficiently selective of the materials that are etched, and they can cause unacceptable damage to a thin protective nickel overcoat. These etchants tend to either etch the nickel overcoat or create pits on the nickel overcoat which thereafter degrade the protective effects.
The present invention overcomes the above-mentioned drawbacks by using dimethyl sulfoxide and a halocarbon compound organic mixture, whereby copper can be selectively etched without affecting other metals such as nickel, chromium, and titanium. In addition, with the solution of the present invention, the etching rate can be precisely modulated by adjusting the ratio of these components based on the desired processing window. This allows for the effective selective removal of copper in a wide variety of commercially important processes in addition to the fabrication of high density interconnects.